The development of limb innervation will be studied in the chick embryo. My general objective is to obtain thorough morphological and functonal descriptions of patterns of neuro-muscular connections under both normal and experimental circumstances. These descriptions will be used to critically evaluate developmental mechanisms of limb innervation. Surgical manipulations of the normal relationship between motoneurons and muscles will be used to alter specific developmental parameters such as limb position, the time or sequence of axon arrival, the number of innervating axons, or the size of the target. Intra-muscular injections of horseradish peroxidase (HRP) will be used to map motoneuron pools supplying individual muscles. Electrophysiological recording techniques will be used to determine the functional connectivity and motor output of abnormally innervated muscles. A comparison of normal limb innervation patterns with limbs whose innervation has been systematically altered can provide important information about developmental mechanisms underlying the formation of specific neuronal connections.